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Plum Tree Diseases

I have a 30+ year old Victoria plum in my garden which has cropped so well for so long it's like a family member now. I'm really worried that it may have 'cytospora canker' or 'bacterial canker'. The trunk has a weeping perforation exuding a trail of sticky gum, and parts of the trunk and branches have longitudinal cracking in the bark. There is some evidence of lichen or fungal growth on some branches, and overall the tree looks rather sorry for itself. I sent 5 photos of the tree, the third (DSC00417) showing the weeping on the trunk. Please could you give an opinion on the matter, and possible course of action?
Thank you for any help you can give me -
Kuib